The invention relates to a process for determining of pressing parameters for pressing of compacts of complex shape and to a device for carrying out such a process.
Generally the condition of a part pressed from basic powder materials and consisting of one or more segments is defined by specifying the dimensions and densities of individual segments of said part. A pressed part as shown in FIG. 1, in the following also called compact, consists for example of three segments, each comprising a segment height hsi with i=1, 2 or 3, important in the course of a powder pressing process, as well as a segment density xcfx81i of each segment. For the compact shown in FIG. 1, not all three segments start on the same basic level, so that in corresponding calculations there have to be taken into consideration also the distances x1 or x3 from a support surface and a basic level, respectively. To simplify the principles explained below, however, there will be regarded in the following a compact, on which all three segments S1, S2 or S3 are extending from a basic level upwards, as this can also be seen in FIG. 2A.
As can be seen in FIG. 2B, an example of a pressing device for pressing basic powder material to said compacts 9 consists in particular of a punch guiding device 1, in which in particular a main punch 2, in the following also referred to as upper ram, is guided in upward and downward direction. On the lower side of the upper ram 2 there are fixed pistons 3, 4 and 5 serving for actuation of individual punches, in the following also referred to as segment punches 6, 7 or 8. The segment punches 6, 7 and 8, can be moved in upward and downward direction relative to the main punch 2 by means of the pistons 3, 4 or 5. The segment punches 6, 7 and 8 are guided in a pressing mould, which here, for the purpose of simplified representation in the drawing, is assumed as identical to the punch guiding device 1. The pressing mould serves for being filled with a basic material in the form of powder or granulate to be pressed, and it ends with a bottom 10 at its lower end. On normal presses, however, there is usually provided instead of such a bottom 10 a comparable arrangement of punches, through which there can be exerted a pressing force from below in direction towards the pressing mould by means of a main punch and/or a plurality of individual segment punches. In the position as shown in FIG. 2B, the arrangement is in pressing position, with the main punch 2 and the segment punches 6, 7 and 8 in lowered position. Hereby the compact 9 is given the shape of the compact shown in FIG. 2A.
In order to achieve in the compact 9 the required segment densities xcfx811, xcfx812 and xcfx813 of the individual segments S1, S2 and S3, respectively, it is necessary before the pressing operation to fill into the respective segments S1, S2 and S3 a considerably larger volume of powder than the volume in pressed condition. Normally the ratio of filling volume to pressed volume for basic powder materials is in the range between 1.8 and 2.3. Having filled in the basic powder material, a pressing force is exerted onto the upper ram 2 by a correspondingly designed pressing device for compacting the basic powder material. Forming of the compact 9 is effected by the segment punches 6, 7 and 8 which are movable independently relative to the upper ram 2 and are moving relative to the main pressing movement of the upper ram or main punch 2.
In order to be able to achieve accurate densities and heights of the individual segments S1, S2 and S3 of the compact 9, the main punch 2 and the segment punches 6, 7 and 8 are provided with distance traveling measuring systems 11-14 measuring the position of the upper ram 2 and of the punches 6-8, respectively. A problem in such pressing devices is the fact that such travel measuring systems 11-14 can not be fixed directly to the platen ends and punch ends, respectively, but are arranged at a more or less long distance at the beginning of the platen and on the upper side of the punches 2, 6-8; respectively.
The arrangement of the travel measuring systems does not have any effect on pressing operations later, but this arrangement does cause problems with respect to the determination of the required pressing parameters. Due to the high pressing pressures applied for pressing the basic powder materials, the individual main punches and segment punches 2, 6-8 will be compressed, too, during pressing. The usual balanced deflection under load of these components of a press is in the range of mm, whereas the accuracy requirements to the compacts are in the range of 0.01 mm.
The usual proceeding for setting of height and density values of the individual segments S1, S2, S3 of a compact 9, comprises a plurality of iterative approximating pressing tests, as a rule considerably more than 15 tests. In one or more first pressing tests, there is initially produced a compact 9 with a density which allows to touch and measure the compact 9. Subsequent it is attempted by iterative pressing tests to set the required height of the parts hs1, hs2 and hs3. Having achieved the recuired heights of the part, the pressing position necessary or this, or pressing height, is defined and set by positive stops for example. Having set or run-in the required heights of the parts hs1-hs3, the density of the part is optimized by usually a plurality of further iterative pressing tests. By adding basic powder material to or taking it away from the respective segment, the density there can be increased or reduced.
If now, for example, the density in segment hs1 is increased by filling in more powder, the corresponding platen or the corresponding segment punch 6 will deflect more during pressing due to the increased pressing force. Consequently the local height of the part hs1 of the segment S1 will change. In addition, the whole pressing device, due to the difference in pressing force, will deflect in a different manner, and this will be transferred also to the segment heights hs2, hs3 of the segments S2 and S3, respectively, and to their densities xcfx812 respectively xcfx813. Consequently also the values of the segments S2 and S3, which are not really affected, have to be newly set in corresponding manner, if parameters have to be changed in the range of segment S1. In other words, due to corresponding interaction, each change of position and/or density in a segment Si will result in necessary changes of the parameters of the remaining segments Si.
To illustrate this, there are shown in FIG. 2B a height hOB from a basic level up to the travel measuring system 11 of the main punch 2, which allows to comprehend the movement of the main punch 2 in upward or downward direction. As explained, the problem is the section of the main punch 2, which in upward and in downward direction is between the travel measuring system 11 and the lower edge of the main punch 2, because this section of the main punch 2 will be compressed in a different way, when applying a first pressing pressure than when applying a different second pressing pressure. The same thing applies to the measuring sections of the individual platens or the arrangements of pistons 3, 4, 5 with segment punches 6, 7 or 8, where the measuring sections h1, h2 or h3 measure always only the distance between the upper edge of a piston and the corresponding travel measuring systems 12-14, but do not measure the differently strong deformations of the sections between the travel measuring systems 12-14 and the corresponding lower edges of the segment punches 6-8, which are varying according to the pressing pressure. These sections, which can not be measured unequivocally with respect to compression, are shown in FIG. 2B by spring symbols Cob, c1-c3.
In one embodiment of a process for determining pressing parameters for the pressing of compacts of complex shape, the process includes determining a density of a first compact having a first height, and setting a predetermined height of the first compact if the first compact has a predetermined nominal density. The compacts are in certain embodiments powder metallurgical or ceramic compacts.
In one embodiment, an automated device for carrying out the process includes a pressing device having a plurality of pressing punches configured to be movable forward and backward in a pressing direction, travel measuring devices configured to measure movements of the pressing punches, a measuring device configured to determine parameters of the compact to determine at least one of a density and a height of a compact, and a calculation device. The calculation device is configured to calculate a filling height for pressing material for a pressing test, whereby the pressing test includes determining a density of a first compact having a first height, and setting a predetermined height of the first compact if the first compact has a predetermined nominal density.
The object of the invention is to propose a process for determining the pressing parameters for pressing compacts of complex shape, in which the number of pressing tests is reduced.
This object is solved by a process with the features of claim 1 or 2, for determining pressing parameters for the pressing of compacts of complex shape, in particular of powder metallurgical or ceramic compacts. An automated device with the features set out in claim 6 allows in an advantageous way the automatic execution of such process.
Advantageously, the total number of pressing tests, which is considerably higher than 15 in a conventional process, may be reduced up to about three or four pressing tests.